The present invention relates to a device for checking the position of a spindle in a machine tool.
The invention has application in machine tools of the type that comprise a bearing structure composed of two longitudinal and parallel slide ways mounted on respective vertical stands, and a beam or gantry traversable on the ways along an axis designated X, supporting a unit that comprises an upright pillar aligned on a vertical axis, of which the bottom end carries a machining head equipped with a spindle to which a tool can be attached.
In general, the pillar can be traversed on the beam horizontally, along an axis designated Y, in a direction transverse to the traversing direction of the beam. The pillar is also slidable vertically along a third axis designated Z, in such a manner that the machining head can be raised and lowered relative to a machining station beneath, which is occupied by a fixture holding a part for machining.
For particular types of operation, finally, the machining head is able to rotate about an axis C parallel to the vertical axis Z, whilst the spindle is also pivotable on an axis A transverse to the selfsame axis Z.
In order to control a machine tool of the type described above, it must be known from one moment to the next exactly where the tool is positioned, during the entire machining process. Using prior art methods, the position of the spindle in space is identified indirectly by optical, mechanical or encoder type systems capable of measuring the x, y and z coordinates of the spindle relative to the point of origin in a Cartesian system established by the X, Y and Z axes and associated rigidly with the fixed structure of the machine tool.
In consequence, the reliability of the measuring system in question, which depends on the connection between fixed and moving parts of the machine, is based on the assumption that the structure will be correctly and stably positioned in relation to the machining station. Similarly, it is assumed that the geometrical positions of the various machine components, namely the slide ways, beam and pillar, will be maintained within prescribed tolerances.
It happens however that the geometrical positions in question cannot always be maintained, and there is also the factor of variations in the structure, caused for example by the effects of load-related heat and deformation, which introduce errors into the measurement.
The object of the present invention, accordingly, is to provide a device for checking the position of a spindle in a machine tool that will be unaffected by the aforementioned drawbacks typical of prior art devices.